Methanol fuel cell comprising a membrane which conducts metal cations

ABSTRACT

The invention relates to a methanol fuel cell comprising a membrane which conducts metallic cations, in which the metallic cations induce the transport of the charge inside the membrane and are advantageously guided in a circuit in the form of a base from the cathode chamber to the anode chamber. The inventive methanol fuel cell prevents the methanol drag associated with proton-conductive membranes, thus producing higher power outputs on a regular basis. A separate transport of the water produced by the reaction is not necessary.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a fuel cell, especially a methanol fuel cell,and to a method of operating this fuel cell.

2. Description of the Related Art

A fuel cell has a cathode, an electrolyte and an anode. The cathode issupplied with an oxidizing agent, for example, air or oxygen, and theanode is supplied with a fuel, for example, hydrogen or methanol.

Various fuel cell types are known, for example, the SOFC fuel cell(SOFC=solid oxide fuel cell) from the publication DE 44 30 958 C1 andthe PEM fuel cell (PEM=proton exchange membrane) from the publication DE195 31 852 C1.

The operating temperature of a PEM fuel cell is about 80° C. A PEM fuelcell can in principle be either an acid or alkaline fuel cell, dependingupon the type of membrane or the working medium. Usually protons areformed at the anode of a PEM fuel cell having a proton conductor in thepresence of the fuel by means of a catalyst. The protons pass throughthe electrolyte and combine at the cathode side with oxygen arising fromthe oxidation medium to water. Electrons are thereby liberated andelectrical energy is generated. The drawback of a methanol fuel cellwith a proton conductor is that the protons, under the influence of theelectric field, in their solvate shells entrain water molecules alongwith them. This electrophoresis effect is associated with a very highdrag factor (number of entrained water molecules per proton). This meanson the one hand that too much water is transported from the anode to thecathode which has a disadvantageous effect on the thermal balance; onthe other hand, methanol is entrained which in general can form a mixedpotential at the cathode and result in a significant reduction in power.

Multiple fuel cells are as a rule connected together electrically andmechanically to produce large electric power utilizing connectingelements. These arrangements are called fuel cell stacks. For the fuel,methane or methanol, among others, can be used. The mentioned fuels areconverted by reformation or oxidation to, among other things, hydrogenor hydrogen-rich gas.

There are two types of methanol fuel cells. The so-called indirectmethanol fuel cell in which initially in a preceding process step ahydrogen-rich gas mixture is produced and which is then led into apolymer electrolyte fuel cell of the usual hydrogen type with anodicplatinum ruthenium catalysts. This process variant is then comprised oftwo stages: gas production and the usual fuel cell. A furthersignificantly simpler variant from the point of view of processtechnology, is the so-called direct methanol fuel cell (DMFC) in whichthe methanol, without intervening stages from the process technologypoint of view, is directly fed to the fuel cell. This cell has incomparison to the first, however, the disadvantage that with a protonconductor as an acidic medium, the direct electrochemical oxidation ofmethanol is a kinetically strongly limited process which, with referenceto a fuel cell, gives rise to considerable loss of cell voltage. Evenwith the best results with the DMFC cells to date these cells hardly canbe expected to compete in classical configurations with the indirectmethanol fuel cell.

This can as a first instance be due to the fact that both the methanolpermeation rate and the water vapor enthalpy in the cathode compartmentare too high in the case of the present day cells. Furthermore, becauseof the unsatisfactory methanol oxidation rate, it is necessary for theoperating temperature of the cell to be significantly above 100° C.There is however no appropriate electrolyte which can remain functionalabove 120° C.

To be economical relative to indirect methanol cells, the DMFC must havevoltages smaller by only 100 mV at the same current densities bycomparison to the indirect cells (with MeOH-permeation) or around 150 mVsmaller without permeation. As simulation results show, the greatestloss originates in anodic overvoltage which derives from the highlyirreversible electrode kinetics. For that reason, the catalyst coatingmust also be uneconomically high; because of the methanol permeation thecathodic catalyst coating must be 10 times higher than is the case withhydrogen cells.

From W. Vielstich: Brennstoffelemente (Fuel elements), Verlag Chemie,1965, P. 73-91 it is known as state of the art to provide an alkalimethanol oxidation in a fuel cell. This method has the advantage, bycomparison to the known acid variant, that the electric chemicalreactions run far more quickly and thus the power of the fuel cell issignificantly higher. In practical applications, KOH is used which isimmobilized by a diaphragm in the fuel cell. It has, however, been knownfor a long time from W. Justi, A. Winsel; Kalte Verbrennung (ColdCombustion), Fuel Cells, Franz Steiner Verlag, Wiesbaden, 1962 that inthis process, carbonate is formed as a drawback and can give rise as arule to plugging up of the diaphragm and to a significant reduction inthe conductivity among other things of the carbonate electrolyte in thediaphragm. Furthermore, problems cannot be excluded in the three-phasezone of the catalytic layer of the fuel cell electrode because ofcarbonate formation.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a fuel cell for theconversion of methanol which is effective and can avoid theaforedescribed drawbacks. It is also an object of the invention toprovide a method of operating such a fuel cell.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 displays a principle of the methanol fuel cell according to theinvention and its method.

DETAILED DESCRIPTION OF THE INVENTION

The methanol fuel cells according to the invention encompass an anodecompartment with an anode, a cathode compartment with a cathode and amembrane disposed between the anode and cathode. This membrane is ametal cation-conducting membrane. Under this designation should beunderstood such a membrane that on its one side accepts metal cationsand on the opposite side makes metal cations available. This effect canbe achieved by ion exchange, diffusion or also by ion conduction. Themembrane advantageously has a good resistance to carbonate. The presenceof carbonate as a rule does not result in a plugging of the membrane andalso does not itself have a negative effect on the conductivity. Bycontrast with proton-conductive membranes, which are known from thestate of the art, the membrane according to the invention conducts metalcations. Suitable metal cations are, for example, Li⁺, Na⁺ or K⁺. Thesemetal cations have relatively small ionic radii within the group ofmetal cations and usually show a high conductivity within the membrane.Under the influence of an electric field, they entrain with them,advantageously, in the membrane, only small amounts of water in the formof solvate shells. Furthermore, the membrane according to the inventionhas only a very limited methanol permeation rate. The membrane accordingto the invention thus advantageously enables positive charge carriertransport without the drawbacks of methanol permeation and the resultingmixed potential formation at a cathode as usually arises in the case ofa proton-conductive membrane.

An advantageous example for a membrane according to the invention is acation exchange membrane, is for example Nafion® or also Neosepta®.These membranes are typically charged with a monovalent alkali metal,for example Li⁺, Na⁺ or K⁺. They have a good conductivity and have highstability against carbonate solutions. For this reason they areespecially suitable for use in an alkali fuel cell.

In an advantageous embodiment of the methanol fuel cell according to theinvention, the methanol fuel cell has means which enables a recycling ofmetal cations from a cathode compartment to an anode compartment or froma cathode to an anode. This means ensures that metal cations, which forexample are released from an anode (anode compartment) travel throughthe membrane to a cathode (cathode compartment) and from there cantravel back again to the anode (anode compartment) (circulation). Suchmeans can be realized by a passage between an anode compartment and acathode compartment. Advantageously, this means according to theinvention gives rise to an internal circulation of the metal ions, i.e.within a fuel cell. The means for recycling of metal cations can howeveralso be provided advantageously between different fuel cells so that theresulting metal cation circulation can encompass a plurality of fuelcells.

In a further advantageous embodiment of the methanol fuel cell accordingto the invention, the cations are available in an alkali solution(base). The means for recycling the metal cations in the simplest caseis a liquid passage which connects the anode compartment and the cathodecompartment with one another. A suitable means is for example a simpletemperature-resistant and corrosion-resistant tube between the anodecompartment and cathode compartment. A supply vessel, pump or inlet oroutlet can optionally be interposed.

A methanol fuel cell stack according to the invention has at least twoand preferably however more fuel cells. In the case in which amultiplicity of methanol fuel cells are connected together according tothe invention in a stack, there can be an internal recycling of themetal cation and as well an external recycle of the metal cations for aplurality of fuel cells is conceivable.

For the internal recycle, there is a circulation path for the metalcations within each individual fuel cell. Upon connection of amultiplicity of methanol fuel cells according to the claims one afteranother, the means for recycling the metal ions can be understood tomean that, for example, metal cations from one anode A1 of one methanolfuel cell BZ1 can pass through the membrane to a cathode K1 and fromthere via a passage to a further methanol fuel cell BZ2. There the metalcations from an anode A2 are fed through a membrane via a cathode K2 tothe anode A1 of the first fuel cell BZ1. In this case, the metal ioncirculation encompasses, for example, two methanol fuel cells of astack. A plurality of fuel cells can also be combined in an optionalmanner.

In the method of operating a methanol fuel cell according to theinvention, during the operation of the fuel cell, metal cations passthrough the membrane from the anode to the cathode. The metal cationsarise at the anode from the alkali oxidation of the fuel, in this casemethanol. From the cathode side the metal cations which are delivered bythe membrane bond with the hydroxyl ions which are provided to a base.This is returned to the anode compartment by suitable means. The metalcations assume the role of charge carrier transports within themembrane. They have, because of their ionic radii by comparison withprotons, a significantly smaller solvate shell so that advantageouslyless water is transported with the cations to the cathode by comparisonwith proton-conducting membranes. The alkali process has the advantageof improved electrochemical conversion at the electrodes so that higherefficiencies are produced normally. Furthermore a separate product waterremoval is not required.

In the method of operating the methanol fuel cell according to theinvention, the metal cations found in the cathode compartment arereturned to an anode compartment. Thus a material-serving circulation ofthe metal cations is enabled.

It is conceivable to provide a direct recycling of the metal cationsfrom a cathode compartment into the anode compartment of a fuel cell aswell as the formation of the circulation by connecting a plurality offuel cells. Advantageously, the recycling of the metal cations iseffected in the form of a base, for example sodium hydroxide orpotassium hydroxide. This has the additional advantage that theprovision of a base both in the anode compartment and in the cathodecompartment increases the electrochemical conversion of the methanol orthe oxygen by comparison with the corresponding reactions in acidmedium.

Below the individual reaction steps of the methanol fuel cell are given(with Na⁺ by way of example as the metal cation):

Anode: CH₃OH+6NaOH→CO₂+6Na⁺+5H₂O+6e⁻

Cathode: 6Na⁺+3H₂O+3/2O₂+6e⁻→6NaOH

Circulation: 6NaOH (Cathode)→6NaOH (Anode)

The reaction schemes satisfy the overall reaction equations for thecombustion reaction of methanol.

CH₃OH+3/2O₂→CO₂+2H₂O

and thus is appropriate to the DMFC reaction process.

A process variant is described below which is based on the use of acirculation with alkali ions. This circulation replaces the previousprocess with product water.

It has been found within the framework of the invention that thedrawbacks described at the outset of the state of the art can be largelyobviated by the use of an alkali DMFC. This has the following advantagesin principle:

The alkali process method is basically not connected with the describeddrag problem. Either alkali ions are transported, i.e. the ionic currentdirection in the membrane is effected in the opposite direction than inthe acid medium, or as in the case of the invention, alkali ions areused in which the drag factor and the methanol permeation rate connectedtherewith is generally held below that in a proton-conductive membrane.

The anodic methanol oxidation is effected by a basic catalyzeddehydrogenation whereby the hydrogen formed itself is electrochemicallyactive. It is therefore to be expected that the overall catalysis runsmore effectively than in acid medium.

The cathodic oxygen reduction in alkali medium is not as stronglylimited as in acid medium. Even here, voltage recovery can be expected.

It is thus possible to eliminate the need for noble metals as catalysts.Raney nickel can be used as electrode material in alkali medium as themethanol electrode. For the oxygen electrode, for example, silver,cobalt or nickel are conceivable as catalysts.

A corresponding process diagram has been shown in FIG. 1. A cationexchange membrane (Nafion®, Neosepta®) charged with a monovalent alkalimetal (Li, Na, K) can be used. These membranes exist already and havebeen especially developed for chloralkali electrolysis so thatstabilization problems or the like do not come into question. TheNafion® membrane has as is known, unusual stability in carbonatesolution and has been used already for that purpose. The electricalconductivity is identical as in the case of chloralkali electrolysis. Itis however significantly less than with proton systems. Exact valuesmust be measured, an estimate gives a factor of 2 to 3.

By comparison to the usual proton conductive DMFC cell, with themethanol fuel cell of the invention, it is necessary to introduce anadditional cathodic-anodic hydroxide circulation. A separate productwater path is not required since the cationic flow is associated with acertain drag factor. To maintain the process, the drag factor of 5/6must be provided at a minimum. With respect to the size of the cation,it is to be expected that significantly less water and thus alsomethanol will be transferred from the anode to the cathode than in theproton-conducting DMFC cell.

What is claimed is:
 1. A method of operating an alkali methanol fuelcell with a metal cation conducting member with the steps of liberatingmetal cations from an alkali solution at the anode, passing the metalcations under the influence of an electric field through the metalcation conducting membrane from the anode to the cathode.
 2. The methodof operating an alkali methanol fuel cell according to claim 1 in whichthe metal cations are returned from the cathode to the anode.
 3. Themethod of operating an alkali methanol fuel cell according to claim 1with the steps of liberating at the anode the metal cations from a basehaving metal cations present at an anode side; forming a base with metalcations traveling through the membrane to the cathode with hydroxyl ionspresent there; returning the base containing the metal cations from thecathode to the anode.
 4. The method of operating an alkali methanol fuelcell stack with at least two methanol fuel cells according to claim 1 inwhich the metal cation in the cathode compartment of a first alkalimethanol fuel cell are fed to the anode compartment of a second alkalimethanol fuel cell.
 5. An alkali methanol fuel cell for carrying out themethod according to claim 1 encompassing an anode compartment with ananode, a cathode compartment with a cathode, and a membrane disposedbetween the anode and the cathode, characterized by a metal cationconducting membrane and means for returning metal cations in the form ofa basic solution from the cathode compartment to the anode compartment.6. An alkali methanol fuel cell according to claim 5 with a liquidpassage between the anode compartment and the cathode compartment as themeans for returning.
 7. A fuel cell stack encompassing at least onealkali methanol fuel cell according to claim 5.